The present invention relates to a process for the preparation of a polyolefin polymer by polymerizing one or more olefin monomers in suspension in the presence of a polymerization catalyst in least one polymerization reactor and a method for controlling the content of hydrocarbons with from 14 to 300 carbon atoms in a polyolefin polymer prepared by polymerizing one or more olefin monomers in suspension.
Suspension polymerization processes in which olefin monomers are polymerized in a liquid medium are well known in the art. The obtained polyolefin polymers are commercially successfully used in a multitude of applications. It is general knowledge that polymerization processes do not produce polymers with uniform molecular weight but with a molecular weight distribution. Accordingly, also suspension polymerization processes produce polymer chains of different length including low-molecular-weight constituents. Such constituents with chain lengths of from 14 to 300 carbon atoms are often designated as oligomers or waxes. While in gas-phase polymerization processes the produced low-molecular-weight polymer chains remain within the growing polymer particles, the low-molecular-weight constituents need special attention in suspension polymerization processes because these constituents are at least at elevated temperatures partially soluble in the suspension medium. This is especially required if a suspension medium is used which is not easily recyclable by vaporization and subsequent condensation.
There exist suspension polymerization processes in which, subsequent to the polymerization, the liquid phase is separated from the polymer particles by vaporizing substantially all of the monomers and the diluent. WO 99/47251 describes for example such a separation process which is carried out in a series of two flash tanks. It is however also common in suspension polymerization processes that the polymer is mechanically removed from the suspension medium after the polymerization reaction, for example by a centrifuge. For cost savings the separated suspension medium, or at least a part of it, is either as such returned to the polymerization reaction or after regeneration, for example by distillation, during which the dissolved low-molecular-weight constituents appear as a paste-like to solid residue which has to be disposed of. Accordingly, it is advantageous if the suspension medium comprises after the separation from the polymer particles as little as possible of waxy components. Moreover, for some applications it can be advantageous for the product properties if as much as possible low-molecular-weight constituents are comprised in the polymer. Consequently, there have been efforts to increase in suspension polymerization processes the portion of low-molecular-weight components which remain with the polymer particles and to reduce the amount of waxy components which have to be disposed of.
EP 0 905 152 A1 describes a process for preparing ethylene copolymers in suspension in which the suspension stream is partly evaporated before removing the suspension medium from the polymer particles, with the result that some of the wax formed in the polymerization is adsorbed onto the polymer particles. However, evaporating a substantial part of the suspension medium uses a high amount of energy and requires introducing reduced pressure to a production plant, which might give rise to safety issues.